You’re standing at the TSA line. Your favorite expensive cologne is in your hand, and you’re staring at the bottle like it’s a ticking time bomb. It says 100 ml. You know the rule is 3.4 ounces. But wait—is it exactly 3.4? Or is it 3.38? If you’re flying out of London, does the "ounce" even mean the same thing?
Converting 100 ml to fl oz seems like a middle school math problem that should have stayed in the classroom, but for travelers and home cooks, it's a high-stakes calculation. Get it wrong at the airport, and your $90 moisturizer ends up in a grey plastic bin. Get it wrong in the kitchen, and your sourdough tastes like a salt lick.
Here is the thing: "fl oz" is a messy term. It isn't just one measurement. Depending on where you are in the world, an ounce changes its identity. If you use the US Customary fluid ounce, 100 ml is approximately 3.38 fl oz. However, if you are using the British Imperial system, that same 100 ml suddenly becomes 3.52 fl oz.
It’s annoying. I know.
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The TSA Rule: Why 100 ml to fl oz is the Magic Number
Most people know the "3-1-1" rule. It’s been the bane of our existence since 2006. The TSA and international aviation authorities like the Civil Aviation Organization (ICAO) settled on 100 ml as the global limit for liquids in carry-ons.
But why do Americans call it the 3.4-ounce rule?
If you do the math precisely, 100 ml divided by 29.5735 (the number of milliliters in a US fluid ounce) gives you 3.3814. Aviation agencies just rounded that up to 3.4 to make it easier for people to remember. It’s a generous rounding, honestly. If they had rounded down to 3.3, half the products on the shelves at Sephora would be illegal to fly with.
Check your bottles. You will notice that almost every "travel size" product is labeled as "3.4 fl oz / 100 ml." Manufacturers do this specifically to satisfy both the metric and imperial markets simultaneously. It is the gold standard of international commerce.
The British Ounce vs. The American Ounce
This is where things get weird. Most people assume an ounce is an ounce. It isn't.
Back in the day, the British Empire decided they wanted their fluid ounces to be based on the volume of one ounce of water. The Americans, being rebellious, stuck with a system based on the old Wine Gallon.
- A US Fluid Ounce is about 29.57 ml.
- An Imperial (UK) Fluid Ounce is about 28.41 ml.
So, if you are looking at a 100 ml bottle in London, it’s technically 3.52 imperial fluid ounces. If you are in New York, it's 3.38 US fluid ounces.
Does this matter for your flight? Not really. Security guards at Heathrow and JFK are both looking for that "100" on the label. But if you’re following a vintage recipe from a British grandmother and you’re using an American measuring cup, your cake is going to be dry. You’ve been warned.
Precise Calculations for the Perfectionists
Sometimes "roughly 3.4" isn't good enough. If you’re working in a lab or mixing skincare ingredients, you need the actual numbers.
To convert 100 ml to fl oz (US), you use the factor of 29.5735.
$$100 / 29.5735 = 3.38140227$$
If you’re working with the Imperial system:
$$100 / 28.4131 = 3.519508$$
Most kitchen scales allow you to toggle between grams and ounces. Since 100 ml of water weighs exactly 100 grams (at standard temperature and pressure), you can actually use a scale to check your volume. It’s often more accurate than those plastic measuring cups with the faded red lines.
Why Do We Still Use Ounces Anyway?
It’s a fair question. Most of the world looks at the United States, Myanmar, and Liberia and wonders why we’re still clinging to a system that requires a calculator for basic tasks.
In the medical field, ounces are essentially dead. If a doctor prescribes 100 ml of a solution, they mean 100 ml. No nurse is going to convert that to 3.38 ounces because the risk of a rounding error is too high. In chemistry, the metric system is king because it’s based on powers of ten. Everything fits together.
But in the consumer world, the ounce is stubborn. It’s "lifestyle" math. We buy 12 oz sodas and 16 oz coffees. 100 ml feels small to an American ear, while 3.4 ounces feels substantial. It’s marketing.
Common Products That Hit the 100 ml Mark
You’d be surprised how many things are exactly 100 ml. It’s not just shampoo.
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- High-end Fragrances: Most "large" perfume bottles are exactly 100 ml. This is by design so you can keep your signature scent in your carry-on.
- Top-shelf Gin: You can often find 100 ml "taster" bottles or gift sets.
- Sunscreen: Many brands now sell "TSA-approved" 100 ml tubes.
- Contact Lens Solution: This is the one that gets people. If your bottle is 120 ml, it doesn't matter if it's half empty. The TSA looks at the capacity of the bottle, not the amount of liquid inside.
If your bottle says 125 ml and it only has a tiny squeeze of lotion left, throw it out or check it. They will take it. They don't care about your logic. They care about the number printed on the plastic.
Pro-Tip: The "Medically Necessary" Loophole
Here is something most people miss: The 100 ml (3.4 oz) limit usually doesn't apply to "medically necessary" liquids.
According to the TSA’s own guidelines, you can bring larger amounts of saline solution, breast milk, or liquid medications. You just have to declare them at the checkpoint. You don't need to jam your 100 ml bottle of contact solution into that tiny quart-sized Ziploc bag if you tell them it’s medical. It can go in its own bin.
Practical Steps for Your Next Trip
Stop guessing. If you’re staring at a bottle and the label is rubbed off, you’re playing a dangerous game with the airport gods.
- Buy a set of silicone travel bottles that are clearly embossed with "100 ml / 3.4 oz." It saves the headache of trying to explain to a tired agent that "it looks like 100 ml."
- Check the label for the 'e' mark. In Europe, you’ll see a small "e" next to the volume (like 100 ml ℮). This is the "estimated sign," meaning the average volume is 100 ml. It's a legal standard that ensures you aren't getting ripped off.
- Weight vs. Volume. Remember that 100 ml of honey is much heavier than 100 ml of water, but they take up the same amount of space. The TSA limit is about volume, not weight.
- Use a Sharpie. If you have a 100 ml bottle and the text is hard to read, circle the "100 ml" with a permanent marker. Make it easy for the security person to say yes to you.
The conversion of 100 ml to fl oz is more than just a math trick; it’s a standard that keeps global travel moving. While 3.38 is the technical answer for Americans, 3.4 is the practical one. Just remember that the container's size is the law, not the liquid level. If you stay at or below 100 ml, you’re golden, regardless of which side of the Atlantic you’re on.